Chemical mechanical polishing (CMP) is already known which removes irregularities from a surface of a semiconductor wafer to flatten the surface. In the case of chemical mechanical polishing, it is necessary to complete the polishing when a film to be polished such as an interlayer insulating film becomes a desired thickness. Moreover, it may be requested to complete polishing when a film to be polished is removed and a base stopper film or barrier film appears like the case of STI (shallow trench isolation) or copper wiring film. As a means for satisfying these requests, a polished state monitoring apparatus is known which detects an end point of chemical mechanical polishing by irradiating a semiconductor wafer by a light-projecting device and detecting a change in reflectivity of a surface to be polished in accordance with the intensity of the light reflected from the wafer in order to prevent excessive or insufficient polishing.
To detect an end point of chemical mechanical polishing, a change in intensity of the light of a single-color light source, such as a semiconductor laser or a light emitting diode (LED), reflected from the polished surface or an optical characteristic such as spectral reflectance of white light reflected from the same may be used. Further, a polished state monitoring apparatus is known which calculates a thickness of a film on a wafer by using the intensity of the light reflected from a semiconductor wafer.
Some of conventional polished state monitoring apparatuses monitor a polished state of a semiconductor wafer, for example, measure a characteristic value such as a thickness by scanning a surface of a semiconductor wafer once every turn of a turntable on which a polishing material is set and sampling at a plurality of points every scan period so as to obtain a characteristic value at each sampling point (region). Specifically, a value obtained by A/D-converting the intensity of the light reflected from the surface of a semiconductor wafer at each sampling point is successively plotted as a characteristic value (Refer to Japanese Laid-open No. 2001-284300). In this case, if a light source is continuously lighted while irradiation light scans the surface of a wafer, the reflection intensity represents a region having a certain length along a scan line. This is also referred to as a sampling point hereafter. In FIG. 1(a), a solid line shows a scan trajectory of irradiation light on a semiconductor wafer and a circle shows a sampling point.
In this case, the scan trajectory on the semiconductor wafer differs every scan because the rotational speed of a turntable on which a polishing material is set is normally different from that of a top ring to which the wafer is attached. For example, as shown in FIG. 1(b), the first, second and third scans performed three consecutive times are performed along different trajectories. Points 1-1, . . . , 1-17, 2-1, . . . , 2-17, and 3-1, . . . , 3-17 are sampling points when performing the sampling 17 times on each scan trajectory.
In many cases, as is well known, a profile of a surface to be polished becomes a shape roughly axis-symmetric to the rotational center of a semiconductor wafer. In monitoring the surface to be polished, if all characteristic values obtained through a plurality of times of scans are mechanically arranged as shown in FIG. 8, a polished state of the surface to be polished at each scan is monitored. However, it is difficult to grasp a polished state at a specific position of a surface to be polished (e.g. wafer center) because characteristic values are changed in each scan operation due to an effect by the above profile. Moreover, there is a problem that the progress of polishing cannot be easily confirmed from a characteristic value because of a difference of a wiring pattern at each sampling point, a difference of a slurry state at each sampling, and the influence of electrical noise or the like.